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Shoreline Changes on the Wave-Influenced Senegal River

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Shoreline Changes on the Wave-Influenced Senegal River water Article Shoreline Changes on the Wave-Influenced Senegal River Delta, West Africa: The Roles of Natural Processes and Human Interventions Mamadou Sadio 1,2, Edward J. Anthony 1,*, Amadou Tahirou Diaw 2, Philippe Dussouillez 1, Jules T. Fleury 1, Alioune Kane 3, Rafael Almar 4 and Elodie Kestenare 4 1 Aix-Marseille University, CEREGE UM 34, Europôle de l’Arbois, 13545 Aix en Provence Cedex 04, France; [email protected] (M.S.); [email protected] (P.D.); fl[email protected] (J.T.F.) 2 Laboratoire d’Enseignement et de Recherche en Géomatique, Ecole Supérieure Polytechnique, Université Cheikh Anta Diop, Dakar, Sénégal; [email protected] 3 Laboratoire de Morphologie et d’Hydrologie, Université Cheikh Anta Diop, Dakar, Sénégal; [email protected] 4 LEGOS (CNRS-IRD-CNES-University of Toulouse), 31400 Toulouse, France; [email protected] (R.A.); [email protected] (E.K.) * Correspondence: [email protected] Academic Editors: Sylvain Ouillon and John W. Day Received: 24 December 2016; Accepted: 12 May 2017; Published: 19 May 2017 Abstract: The Senegal River delta in West Africa, one of the finest examples of “wave-influenced” deltas, is bounded by a spit periodically breached by waves, each breach then acting as a shifting mouth of the Senegal River. Using European Re-Analysis (ERA) hindcast wave data from 1984 to 2015 generated by the Wave Atmospheric Model (WAM) of the European Centre for Medium-Range Weather Forecasts (ECMWF), we calculated longshore sediment transport rates along the spit. We also analysed spit width, spit migration rates, and changes in the position and width of the river mouth from aerial photographs and satellite images between 1954 and 2015. In 2003, an artificial breach was cut through the spit to prevent river flooding of the historic city of St. Louis. Analysis of past spit growth rates and of the breaching length scale associated with maximum spit elongation, and a reported increase in the frequency of high flood water levels between 1994 and 2003, suggest, together, that an impending natural breach was likely to have occurred close to the time frame of the artificial 2003 breach. Following this breach, the new river mouth was widened rapidly by flood discharge evacuation, but stabilised to its usual hydraulic width of <2 km. In 2012, severe erosion of the residual spit downdrift of the mouth may have been due to a significant drop (~15%) in the longshore sand transport volume and to a lower sediment bypassing fraction across the river mouth. This wave erosion of the residual spit led to rapid exceptional widening of the mouth to ~5 km that has not been compensated by updrift spit elongation. This wider mouth may now be acting as a large depocentre for sand transported alongshore from updrift, and has contributed to an increase in the tidal influence affecting the lower delta. Wave erosion of the residual spit has led to the destruction of villages, tourist facilities and infrastructure. This erosion of the spit has also exposed part of the delta plain directly to waves, and reinforced the saline intrusion within the Senegal delta. Understanding the mechanisms and processes behind these changes is important in planning of future shoreline management and decision-making regarding the articulations between coastal protection offered by the wave-built spit and flooding of the lower delta plain of the Senegal River. Keywords: Senegal River delta; Langue de Barbarie spit; delta vulnerability; river-mouth migration; spit breaching; ERA hindcast waves; longshore sediment transport Water 2017, 9, 357; doi:10.3390/w9050357 www.mdpi.com/journal/water Water 2017, 9, 357 2 of 17 1. Introduction The impacts of human activities on coasts are often accompanied by a lack of understanding of the consequences of these activities on the hydrodynamic and sediment redistribution processes that shape coasts [1,2]. Alongshore sediment transport, gradients in transport, and interception of drifting sediment by natural or artificial (man-made) boundaries, including river mouths and inlets, are, from a coastal management point of view, very important, as these processes are significant drivers of short-term to medium-term (days to years) shoreline change. Much of the West African coast (Figure1) is wave-dominated, and is classified as a cyclone- and storm-free “West Coast Swell Environment” in the global wave classification scheme of Davies [3], with a subsidiary contribution from shorter-period trade-wind waves from the Atlantic. In Figure1, continental shelf width (clearer hue along the coast) is a fine indicator of the distribution of long stretches of wave-dominated coast (narrow shelf) and the much more limited, predominantly tidal, estuarine sector between Sierra Leone and Guinea-Bissau (broad, low-gradient shelf), subject to significant wave energy dissipation [4,5]. The West African coast is also characterised by a plethora of river deltas, the largest of which are those of the Niger, Senegal and Volta (Figure1). Abundant sand supplies and strong wave-induced longshore drift have favoured the construction of numerous sand barriers, including at the mouths of these three deltas. These barriers are major settlement sites on the coast as they provide higher-lying areas above lagoons and wetlands, while acting as valuable aquifers. On the coast of Senegal, the barriers are generally elongate to curvilinear spits formed at the mouths of tidal or fluvial ria-like embayments. These spits are commonly capped by dunes, but individual beach ridges are visible in some of the more southern ones. These spits have a protective role on the back-barrier wetlands and lagoons by buffering wave energy. By forming alongshore barriers, they are also important in the regulation of the freshwater-saltwater balance and ecology of these lagoons and back-barrier wetlands, both of which can be considerably altered by breaches in the spits or by spit erosion [6,7]. This is particularly the case of the largest of the Senegal wetland systems, that of the lower Senegal River and delta plain (Figure2). The Senegal River delta is an iconic example of a delta subject to strong wave action [8–12]. This delta is often represented in the ternary (river-wave-tide) classification of Galloway [13] at the wave apex. Using a fluvial dominance ratio—defined as river sediment input versus the potential maximum alongshore sediment transport away from the delta mouth—to quantify the balance between river inputs and the ability of waves to spread sediments along the coast, Nienhuis et al. [12] computed a value of 0.04 for the Senegal, which highlights the strong role of wave-induced longshore transport along this delta’s shoreline. A manifestation of this strong longshore transport potential is a long narrow sand spit presently fronting the delta plain, the Langue de Barbarie [10]. This spit has historically played an important role not only in the protection of the lower Senegal delta plain but also in regulating saltwater intrusion by diverting the mouth of the river several kilometres southwards. Of particular significance, in terms of long-term flood-risk and coastal management, is the historic and picturesque city of St. Louis (population in 2013: 300,000), a UNESCO world heritage site located in the proximal part of the delta (Figure2). The cultural attractiveness of St. Louis, a French colonial city, and the biodiversity of the deltaic wetlands and lagoon bound by the Langue de Barbarie spit have also generated a substantial rise in tourism. Much of St. Louis, which has undergone a rapid growth in population over the last 50 years, lies at an elevation of less than 2.5 m above sea level [14], and the city has, therefore, been prone to the flooding that affects the lower Senegal valley in the rainy season (May to October). In October 2003, to avoid flooding of St. Louis in the wake of a massive rise in the water discharge of the Senegal River, an artificial breach was hastily cut through the Langue de Barbarie, generating rapid reworking of the spit. In the present paper, we describe the recent dynamics of the spit within the framework of development of the Senegal delta and specifically aim at disentangling processes of natural forcing from those of the impact of this breach. Two approaches are used in the study: (1) clearly define the wave climate and longshore sediment transport potential along the Langue de Barbarie; and (2) compare spit behaviour patterns prior to, and following the October 2003 artificial Water 2017, 9, 357 3 of 17 breach.Water 2017 Both, 9, 357 of these approaches are important in understanding the current dynamics prevailing3 of 17 Water 2017, 9, 357 3 of 17 alongprevailing this deltaic along coast. this Theydeltaic should coast. alsoThey be should of use also in planning be of use of futurein planning shoreline of future management shoreline and prevailing along this deltaic coast. They should also be of use in planning of future shoreline decision-makingmanagement and regarding decision-making the articulations regarding between the articulations coastal protection between coastal offered protection by the wave-built offered by spit management and decision-making regarding the articulations between coastal protection offered by andthe flooding wave-built of the spit lower and flooding delta plain of the of lower the Senegal delta plain River. of the Senegal River. the wave-built spit and flooding of the lower delta plain of the Senegal River. Figure 1. The coast of West Africa, showing the Senegal (box) and other major river deltas. Much of FigureFigure 1. The1. The coast coast of of West West Africa, Africa, showing showing thethe SenegalSenegal (box) and and other other major major river river deltas. deltas. Much Much of of this coast is wave-dominated, and is characterised by beach-ridge sand barriers and spits.
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